A marine seismic streamer is an elongate cable-like structure, (typically up to several thousand meters long), which contains arrays of hydrophones and associated electronic equipment along its length, and which is used in marine seismic surveying. In order to perform a 3D/4D marine seismic survey, a plurality of such streamers are towed behind a seismic survey vessel. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones, and then digitized and processed to build up a representation of the earth strata in the area being surveyed.
The streamers are typically towed at a constant depth of about ten meters, in order to facilitate the removal of undesired “ghost” reflections from the surface of the water. To keep the streamers at a constant depth, control devices known as “birds” are attached to each streamer at intervals of 200 to 300 meters.
Low frequency depth variations and lateral motions of towed streamers are inevitable. The main reasons for streamer depth variations are long periodic waves. In general, the worst-case situation is when towing in the same direction as the swell. Streamer lateral motions are mainly due to sea current components perpendicular to the towing direction. In the case of both swell and cross-current influences, the risk of streamer entanglement is therefore increased.
The streamer tension decreases proportional to the distance from the towing point. Therefore, low frequency streamer lateral and vertical motion tends to have larger amplitudes closer to the tail. However, the forces acting perpendicular to the streamer are non-uniformly distributed over the streamer length, and change over time as the towed array moves forward.
During a seismic survey, the streamers are intended to remain straight parallel to each other and equally spaced. However, after deploying the streamers, it is typically necessary for the vessel to cruise in a straight line for at least three streamer lengths before the streamer distribution approximates this ideal arrangement and the survey can begin. This increases the time taken to carry out the survey, and therefore increases the cost of the survey. However, because of sea currents, the streamers frequently fail to accurately follow the path of the seismic survey vessel, sometimes deviating from this path by an angle, known as the feathering angle. This can adversely affect the coverage of the survey, frequently requiring that certain parts of the survey be repeated. In really bad circumstances, the streamers can actually become entangled, especially at the end of the streamers, which can cause great damage and considerable financial loss.
Several patents cover steering devices for seismic streamers.
U.S. Pat. No. 5,443,027 (Norman, L. Owsley et al.) describes a lateral force device for displacing a towed underwater acoustic cable, providing displacement in the horizontal and vertical directions, the device having a spool and a rotationally mounted winged fuselage.
U.S. Pat. No. 6,011,752 (Loran, D. Ambs et al.) describes a seismic streamer position control having a body with a first end and a second end and a bore therethrough from the first end to the second end for receiving a seismic streamer therethrough, at least one control surface, the at least one recess in which is initially disposed the at least one control surface movably connected to the body for movement from and into the at least one recess and for movement, when extended from the body, for attitude adjustment.
U.S. Pat. No. 6,144,342 (Bertheas, Jean et al.) describes a method for controlling the navigation of a towed linear acoustic antenna. Birds equipped with variable-incidence wings are fixed onto the antenna. Through a differential action, the wings allow the birds to be turned about the longitudinal axis of the antenna so that a hydrodynamic force oriented in any given direction about the longitudinal axis of the antenna is obtained. The bird can be detached automatically as the antenna is raised so that the antenna can be wound freely onto a drum. This method also allows full control of the deformation, immersion and heading of the antenna.
WO 03/008906 A2 (Nicholson, James et al.) describes a control device for controlling the position of a marine streamer comprising an annular aerofoil, a mount for mounting the aerofoil onto and around the streamer, and control means for controlling the tilt and/or rotation of the annular aerofoil to thereby adjust the lateral position and/or depth of the streamer.
U.S. Pat. No. 6,671,223 B2 (Bittleston, Simon Hastings) describes a control device or “bird” for controlling the position of a marine seismic streamer, which is provided with an elongate, partly flexible body which is designed to be electrically and mechanical connected in series with a streamer. In a preferred form the bird has two opposed wings which are independently controllable in order to control the streamer's lateral position as well as depth.
U.S. Pat. No. 6,879,542 B2 (Soreau, Didier) describes a bird with a pair of horizontal wings and a pair of vertical wings for lateral and horizontal control respectively. The wing rotation is controlled by pendulum forces on lateral force wing pair.
U.S. Pat. No. 6,459,653 (Kuche, Hans-Walter) describes a collapsible depth controller for mounting in relation to seismic cables or similar devices, comprising two wings, each being rotatably connected to a main body relative to a first axis of rotation perpendicular to the longitudinal direction of the depth controller.
U.S. Pat. No. 5,619,474 (Kuche, Hans-Walter) describes a depth control apparatus for seismic streamers, comprising a central unit adapted for inserted mounting in the streamer substantially in axial alignment with the streamer, a housing which can rotate about the central unit and carries at least one control wing being angularly adjustable about a transverse axis adapted to be substantially horizontal in operative position, and an electric motor for adjusting the angle of the control wing about the transverse axis.
U.S. Pat. No. 6,016,286 (Olivier, Andre W. et al.) describes a depth control device for an underwater cable which includes a rotary actuator having a rotating output shaft connected to a swash plate. Rotation of the swash plate by the actuator causes a rocker arm to pivot about an axis transverse to the axis of the output shaft.
U.S. Pat. No. 6,525,992 (Olivier, Andre W. et al.) describes a device for controlling the position of an underwater cable comprising a body, first and second actuators, and a pair of wings. The body is stationary mountable to the underwater cable and the first and second actuators are disposed in the body. Each wing has an axis of rotation and the wings are coupled to the first and second actuators to control the depth and the horizontal position of the underwater cable in the water.
Birds in accordance with these current designs suffer from a number of disadvantages. Because the birds (in some of the patents mentioned above) hang beneath the streamer, they produce considerable noise as they are towed through the water. This noise interferes with the reflected signals detected by the hydrophones in the streamers. Some of the birds comprise a pair of wings or rudders mounted on a rotatably structure surrounding the seismic steamer in order to generate lift force in a specified direction. This is an expensive and relatively complex electro-mechanical construction that is highly vulnerable in underwater operations. Finally, some of the birds mentioned in the patents above operate with a pair of wings or rudders in a bank-to-turn manoeuvre. This adds complexity to the local control loop, and also slows down the response time of the overall streamer positioning control system.